Fabry disease is a glycosphingolipid (GSL) lysosomal storage disorder resulting from an X-linked inherited deficiency of lysosomal α-galactosidase A (α-GAL), an enzyme responsible for the hydrolysis of terminal α-galactosyl residues from glycosphingolipids (Brady et al. N Engl J. Med. 1967; 276: 1163-7). A deficiency in the enzyme activity results in a progressive deposition of neutral glycosphingolipids, predominantly globotriaosylceramide (ceramide trihexoside, CTH, GL-3), in cell of Fabry patients. Symptoms can be severe and debilitating, including kidney failure and increased risk of heart attack and stroke. Certain of the mutations cause changes in the amino acid sequence of α-GAL that may result in the production of α-GAL with reduced stability that does not fold into its correct three-dimensional shape. Although α-GAL produced in patient cells often retains the potential for some level of biological activity, the cell's quality control mechanisms recognize and retain misfolded α-GAL in the endoplasmic reticulum, or ER, until it is ultimately moved to another part of the cell for degradation and elimination. Consequently, little or no α-GAL moves to the lysosome, where it normally hydrolyzes GL-3. This leads to accumulation of GL-3 in cells, which is believed to be the cause of the symptoms of Fabry disease. In addition, accumulation of the misfolded α-GAL enzyme in the ER may lead to stress on cells and inflammatory-like responses, which may contribute to cellular dysfunction and disease.
Fabry disease is classified by clinical manifestations into three groups: a classic form with generalized vasculopathy, an atypical variant form with clinical manifestations limited to cardiac tissue, and later-onset disease, which includes female carriers with mild to severe forms of the disease.
The frequency of the classical form of disease is estimated to be about 1:40,000 to 1:60,000 in males, and is reported throughout the world within different ethnic groups. Classically affected males have little or no detectable α-GAL levels and are the most severely affected. The clinical manifestations include angiokeratoma (small, raised reddish-purple blemishes on the skin), acroparesthesias (burning in hands and feet), hypohidrosis (decreased ability to sweat), and characteristic corneal and lenticular opacities (The Metabolic and Molecular Bases of Inherited Disease, 8th Edition 2001, Scriver et al., ed., pp. 3733-3774, McGraw-Hill, New York). Lipid storage may lead to impaired arterial circulation and increased risk of heart attack or stroke. The heart may also become enlarged and the kidneys may become progressively involved. Other symptoms include fever and gastrointestinal difficulties, particularly after eating. The affected male's life expectancy is reduced, and death usually occurs in the fourth or fifth decade as a result of vascular disease of the heart, brain, and/or kidneys.
Individuals with later-onset Fabry disease can be male or female. Late-onset Fabry disease presents as the atypical variant form, and growing evidence indicates there may be a significant number of “atypical variants” which are unaccounted for in the world. Females, who inherit an X chromosome containing an α-GAL mutation, may exhibit symptoms later in life, significantly increasing the prevalence of this disease. These patients typically first experience disease symptoms in adulthood, and often have disease symptoms focused on a single organ. For example, many males and females with later-onset Fabry disease have enlargement of the left ventricle of the heart. Later-onset Fabry disease may also present in the form of strokes of unknown cause. As the patients advance in age, the cardiac complications of the disease progress, and can lead to death.
In contrast, patients with the milder “cardiac variant” of Fabry disease normally have 5-15% of normal α-GAL activity, and present with left ventricular hypertrophy or a cardiomyopathy. These cardiac variant patients remain essentially asymptomatic when their classically affected counterparts are severely compromised. Cardiac variants were found in 11% of adult male patients with unexplained left ventricular hypertrophic cardiomyopathy, suggesting that Fabry disease may be more frequent than previously estimated (Nakao et al., N. Engl. J. Med. 1995; 333: 288-293).
The α-GAL gene has been mapped to Xq22 (Bishop et al., Am. J. Hum. Genet. 1985; 37: A144), and the full-length cDNA and entire 12-kb genomic sequences encoding α-GAL have been reported (Calhoun et al., Proc. Natl. Acad. Sci. USA. 1985; 82: 7364-7368; Bishop et al., Proc. Natl. Acad. Sci. USA. 1986; 83: 4859-4863; Tsuji et al., Eur. J. Biochem. 1987; 165: 275-280; and Kornreich et al., Nucleic Acids Res. 1989; 17: 3301-3302). There is a marked genetic heterogeneity of mutations that cause Fabry disease (The Metabolic and Molecular Bases of Inherited Disease, 8th Edition 2001, Scriver et al., ed., pp. 3733-3774, McGraw-Hill, New York; Eng et al., Am. J. Hum. Genet. 1993; 53: 1186-1197; Eng et al., Mol. Med. 1997; 3: 174-182; and Davies et al., Eur. J. Hum. Genet. 1996; 4: 219-224). To date, a variety of missense, nonsense, and splicing mutations, in addition to small deletions and insertions, and larger gene rearrangements, have been reported, although the majority of mutations are missense mutations.
Fabry disease is heterogeneous and it is often difficult to correlate genotype with phenotype. People with the same genotype often exhibit different clinical symptoms and disease pathology. However, there appears to be a correlation between residual enzyme activity and disease severity, with the lower the α-GAL activity resulting in the greatest severity of disease. Although the vast majority of α-GAL mutations are missense mutations, with most being outside the catalytic site, it difficult to predict which mutations result in an unstable enzyme that could be “rescued” by a specific pharmacological chaperone (SPC) which stabilizes the enzyme, and which ones cannot be stabilized using a SPC.